A large number of power electronic units require that the alternating current line voltage be converted to a direct current. For that purpose, power supplies in the form of switched converters and inverters have been provided in the prior art. Such circuits usually involve a large storage capacitor connected across a rectifier bridge output, and this causes the input line current to become highly non-sinusoidal. Accordingly, poor effective power factors are encountered, which results in higher input current for a given power output of the power supplies.
Specifically, a conventional switched power supply operates as a capacitive input load for the rectified line voltage. Such a load draws relatively high currents near the peaks of the alternating current cycle, and substantially zero current for the remainder of the cycle. This results in a poor power factor, which manifests itself as a larger RMS current for a given input voltage and power load, than if the load were purely resistive. A typical prior art switched power supply, for example, draws about 1.5 times the RMS current for a given power output, as compared with a purely resistive load.
Consequently, for a given line current the usual prior art switched power supply is capable of delivering only about two-thirds the power which could be delivered if the line current were sinusoidal, which is the case when the load is purely resistive.
The present invention provides a switched-mode power supply which includes a boost converter between a diode rectifier bridge and the storage capacitor included in the power supply. The boost converter is incorporated into the power supply because (1) it draws a relatively smooth current from the line, and (2) it permits the voltage on the storage capacitor to be higher than the voltage produced by the diode rectifier bridge which provides for more efficient energy storage. The boost converter, as will be described, has an inductance coil in its input circuit and a capacitor in its output circuit, both being desirable for the purposes of the present invention, which will become apparent as the description proceeds.
The boost converter in the power supply of the invention is constructed to draw a sinusoidal current by operating as a current regulator with a current reference control signal set to track the line voltage waveform to control the voltage across the aforesaid storage capacitor. The voltage across the storage capacitor is achieved by controlling the magnitude of the current reference control signal, but not its wave shape, by a regulating signal.
By regulating the voltage across the storage capacitor at a high level, excellent power hold-up time can be achieved when line power is removed, regardless of the input line voltage, with much less storage capacitance being required as compared with the storage capacitance requirements of the prior art switched power supplies.
The circuit and system of the present invention is of the same general type as described in an article entitled "Sinusoidal Line Current Rectification with a 100 KHz B-sit Step-Up Converter"; by Ned Mohan et al (Page 92 of a publication by the I.E.E.E. dated 1984).